The present invention relates generally to medical devices, and more particularly to, a neuromuscular transmission monitor simplified scoring and a method of neuromuscular transmission scoring.
Neuromuscular monitoring is the measurement of the neuromuscular transmission of a patient. Before surgery, anesthesia is administered to the patient. A part of anesthesia is the administration of a neuromuscular blocking agent. This agent is used to block neuromuscular activity and paralyze the muscles of the body to allow intubation and/or to facilitate the surgical procedure. Intubation is the process of placing a tube into the patient""s trachea to establish an airway. The tube is connected to a ventilator which effectively breathes for the patient. Inserting the tube requires that the muscles of the throat, diaphragm, and larynx must be paralyzed. Without this paralyzing effect, reflexes would make the insertion of the tube difficult and problematic.
Additionally, certain surgical procedures are extremely delicate and must be performed under a microscope. In such cases, any patient movement could be disastrous to the outcome of the procedure. Coughing, bucking, or any reflex movement due to the irritation caused by the tube, for example, is a common occurrence in unblocked, or unparalyzed patients. To eliminate all movement, a large dose of neuromuscular blocking agent is given to completely paralyze the patient. Further, abdominal cases generally require the patient""s muscle tone be totally abolished to facilitate the surgical procedure.
At the end of a surgical procedure, the neuromuscular block must be reversed and neuromuscular activity must be returned to normal to ensure the patient is able to breathe unassisted before the ventilator and tube are removed. As is evident then, it is necessary to measure neuromuscular response and effectively assess those results. The measurement of neuromuscular transmission generally involves electrically stimulating a nerve fiber and measuring the physical response of the associated muscle. Typically, the stimulation occurs at the ulnar nerve near the wrist. In response, the adductor pollicis, near the thumb, moves responsively. Depending on the amount of blocking agent administered, different degrees of neuromuscular block can be achieved.
Some cases require very little paralyzation, while others require long periods of intense block. After applying a small electrical current to the patient""s skin near the ulnar nerve, the response of the muscle in the thumb is recorded. With a deep block, the thumb may not move at all. With no block, the thumb""s movement is quite pronounced. With a shallow block, the thumb""s movement is somewhere therebetween. A trained anesthesiologist can gauge the thumb movement by feel and adjust the administration of the drug accordingly. While this method is quite effective for a trained anesthesiologist, it is dependent on the anesthesiologist""s skill level and provides no quantitative recorded data.
The use of neuromuscular transmission monitors to measure neuromuscular transmission results in a wide dynamic range in which the complete spectrum is comprised of three segments, each segment having a unique set of parameters applicable to its own scale. The least sensitive segment is the Train-of-Four (TOF). In this technique, a constant current waveform of four pulses is applied. This method allows determination of the block depth to be made independent of the absolute response amplitude. The absolute amplitude can vary with temperature, arm position, stimulus current and other variables. The response in the TOF technique is four muscular movements corresponding to the four stimulus pulses. The ratio of the fourth response amplitude to the first response amplitude is the TOF ratio. This scale ranges from 0 to 100% and is used when the patient is lightly blocked or during recovery from neuromuscular block. The next segment, or scale, is the Twitch Count (TC) which is a medium sensitivity segment used where there is a moderate degree of paralyzation. The Twitch Count is the actual number of responses from a series of four pulses. The Twitch Count can range from 1 up to 4. The most sensitive segment is the Post-Tetanic Count (PTC) and is measured using a more aggressive stimulus. This method applies a more intense stimulus to the nerve to produce a condition of tetany in the corresponding muscle. This has the effect of sensitizing the neuromuscular junction. Immediately following this intense stimulus, a periodic stimulus is delivered. The number of responses to the periodic stimulus is the PTC value. The PTC value can range from 0 up to 20. A fourth neuromuscular transmission mode is sometimes used in critical situations. This mode measures the normalized twitch amplitude rapidly to determine when intubation is possible.
Each of these various different scales can be used during a typical case to measure neuromuscular transmission response of a patient. Since the values are not progressive and since there is some overlap between the scales, they are not well understood by most clinicians. This results in general confusion about these scales, their readings, and how they relate to one another. For example, it is not obvious that a patient with a PTC of 5 is more blocked than a patient with a Twitch Count of 3, or that a patient with a TOF ratio of 50% is less blocked than one with a Twitch Count of 1.
The problem is best described with an analogy to an automobile having three speedometers ranges. One will readily recognize the confusion to most automobile operators if one speedometer, meant for low speeds, reports the speed in feet per second, while a second speedometer, meant for moderate speeds, reports the speed in kilometers per hour, and a third speedometer, designed for high speeds, reports the speed in miles per hour. Assume further that the driver must know which speedometer to choose at any given time and if he is to maintain his speed at a given limit, the driver may have to make a units conversion xe2x80x9con the flyxe2x80x9d.
During neuromuscular monitoring of a patient under anesthesia, it is-also sometimes important to the anesthesiologist to trend the amount of blocking at given times. Having three different scales of measurement makes it very difficult to trend from anesthesia administration, through surgery, and through the administration of reversal agents.
It is therefore evident that there is a need for a simplified neuromuscular transmission scoring system.
The present invention includes a method and apparatus that incorporates a simplified neuromuscular transmission scoring system that solves the aforementioned problems.
The present invention consolidates the multiple, different scales of measurement for neuromuscular transmission scores into a single universal continuum. In addition, the monitor is capable of determining the appropriate mode of operation. The invention includes the implementation of a simple, single scale to measure neuromuscular transmission. The measuring device applies the correct stimuli and acceptable techniques to measure the TOF ratio, the Twitch Count, and the PTC. The measured data is converted into a single scale or neuromuscular universal score. The single universal scale is easy to learn, use, and display trending. The complexity of the multiple different conventional scales then need not be revealed to the clinician to avoid confusion.
In accordance with one aspect of the invention, a method of neuromuscular (NM) transmission scoring includes applying a NM stimuli to a patient, measuring a NM response from the patient, and assigning a universal value of the NM response to a single progressive scale that encompasses at least 2 stimulus modes, each of the stimulus modes having a unique scale and parameter definition.
In accordance with another aspect of the invention, a method of converting neuromuscular transmission measurement scales to a single progressive scale, additionally includes determining which one of the at least 2 different scales the neuromuscular response belongs thereto, and then assigning a value of the neuromuscular response within the one of the at least 2 different scales. The assigned value is then converted to the single progressive scale encompassing each of the at least 2 different scales.
In accordance with another aspect of the invention, a computer program is disclosed that resides on a computer readable memory capable of causing a processor, when executed, to receive an NM response value that is in one of at least two different formats and then determine which one of the at least two different formats the NM response value is in. The computer program then converts the NM response value to a universal value applicable to a single scale encompassing each of the different formats.
In accordance with yet another aspect of the invention, a neuromuscular transmission monitor is disclosed that has at least one patient electrode to stimulate a muscle of a patient and a transducer to measure an NM response to the muscle stimuli and create a NM response signal therefrom. A power supply is connected to the patient electrodes to supply muscle stimulating power to the patient electrode. A processing unit is connected to the patient electrode and a transducer to control the muscle stimulating power to the patient electrode and process the neuromuscular response signal from the transducer. The processing unit is further programmed to determine a correct stimulus mode for the neuromuscular response signal and produce a non-mode specific value applicable to a single scale that encompasses multiple stimulus modes based on the determined stimulus mode and the neuromuscular response signal.
Various other features, objects and advantages of the present invention will be made apparent from the following detailed description and the drawings.